Airplane landing wheel brake control apparatus



April 28, 1953 G. w. YARBER r-:T A1. 2,536,700'

AIRPLANE LANDING WHEEL BRAKE CONTROL` APPARATUS Filed Feb. 21, 195oRUS/i F CHASE MM im A fro/@NE YS Patented Apr. 28, 1953 AIRPLANE LANDINGWHEEL BRAKE CNTROL APPARATUS Gordon W. Yarber, near Seattle, and HarryH.

Howell and Rush F. Chase, Seattle, Wash., assignors to Boeing AirplaneCompany, Seattle, Wash., a corporation of Delaware Application February21, 1950, Serial No. 145,368

, 19 Claims. l

This invention relates to automatic brake oo ntrol apparatus forpreventing skidding of an airplanes landing wheels by application ofexcessive brake pressure. Reference is made to the copending applicationof Gordon W. Yarber, Serial No. 90,965, led May 2, 1949, disclosingautomatic brake control mechanism for a similar purpose. A broad objectof our present invention is to provide automatic brake control apparatuswhich will operate effectively under the Widely diverse airplane landingconditions which may arise and affect the control requirements.

Automatic control of braking is especially advantageous in the case oflarger type airplanes. It is normally diflicult for a pilot to bring alarge airplane to a stop consistently in the shortest achievable landingrun by most efficient use of wheel brakes, because he cannot readilysense skidding and also because of the human reaction time element.Skidding causes wear and frequently failure of tires. Also, groundlooping can result if the airplane enters an uncontrolled skid.Moreover, a skidding wheel actually produces less frictional resistance,hence less deceleration of the aircraft than a wheel braked just to theverge of skidding, for example.

Wheel deceleration and acceleration detecting mechanism is disclosed inthe copending application cited above. Our present invention utilizesbasic mechanism, preferably of that type, although it is directedprimarily to an entire automatic brake control system or apparatus, inwhich the detecting mechanism is but an element. As in the copendingapplication, such detecting mechanism actuates follow-up or controlmeans,

preferably comprising an electric circuit, operable to relieve brakepressure in response to detection of the beginning of a skid. When thusreleased, the wheels recover speed, and when that occurs braking isautomatically restored. Such removal and restoration of braking pressuremay occur intermittently a number of times during a landing run and maybe accomplished in the usual case involving hydraulically-operatedbrakes by energizing and deenergizing a solenoid valve located in thehydraulic brake fluid supply line.

The detecting mechanism referred to above includes a wheel-rotatedmember or flywheel which is spun by rotation of the airplane landingwheels when they iirst contact the ground. Should the wheels tend toskid thereafter the resultant drop in wheel speed below the speed of theflywheel acting as a speed reference, actuates skid-detector electriccontacts, which in turn actuate the control circuit. The latterenergizes the valve which removes pressure-fluid from the brakemechanism, the same being accomplished before the skid develops beyondits in` cipient stage. As a result, complete skidding, that is lockingof the landing wheels by excessive brake pressure, is automaticallyprevented, and instead an average braking pressure is attained,corresponding to that which would keep the wheels substantially at theverge of skidding.

Our present invention chiefly pertains to improvement modifications orcontrol renements implemented by a novel control circuit or equivalentmeans for converting signals from. the detector mechanism into controloperation of the hydraulic valve or similar brake-releasing device in asystem of the type mentioned.

An object is to improve the reliability of the automatic control, andmore particularly to remove a former limitation therein resulting :fromthe requirement that the reference iiywheel be initially spun by thelanding wheels in order .to condition the detector mechanism forskid-detecting operation. If the pilot accidentally locked the landingwheels before and atinitial ground contact, they would skid immediatelyand the flywheel would not be rotated. Such a skid would remainundetected.

Accordingly,` the improved automatic control means or circuit includes acircuit interlock which is conditioned before landing and insures thatthe landing wheels, hence the flywheel, will` be rotated as the airplanelands, independent of errors of judgment of the pilot in operating thebrake controls as mentioned above. The interlock effectively makes itimpossible for the pilot to lock, and thereby skid the wheelsaccidentally at the instant of initial ground contact.

A further and related object is to insure removal o1' disablement of theinterlock, and restoration of automatically controlled braking as soonas practicable after initial ground Contact. Accordingly, the controlcircuit includes interlock release means responsive to actuation of thewheelspeed-recovery detector, such detector responding to initialspinning ofthe landing wheels as well as to speed recovery accelerationthereof following release of the brakes when the wheels tend to skidduring the ensuing landing run.

A further object of the invention is to prevent premature removal of theinterlock and restoration of brake pressure, such as in thecase of afaulty or bounce landing. This is desirable because if the ilywheelretards suiiiciently of its own accord in the interval betweensuccessive bounces,

. for instance, to the point where its speed is no longer sunicient toenable detecting a skid, then, as in the case of locked wheels uponinitial ground contact, over-application of brake pressure by the pilotcould bring on a dangerous skid which would develop undetected.

In light of this object the control circuit includes holding meansoperable to prolong the effectiveness of the interlock for adeterminable short period of time immediately following initialactuation of the wheel-speed-recovery detector. The period ofprolongation has a maximum set length, but it can be shortened andcontrolled braking established correspondingly sooner by a further orcontinuing signal from the wheel-speed-recovery detector, asin the caseof a perfect landing.

Still another and important object is to provide automatic anti-skidcontrol which imposes its eifect upon the pilot-controlled brakingsystem in a manner conducive to maximum braking eioiency, It isimportant, for example, that no time be lost between recovery of wheelspeed after detection and elimination or a tendency to skid, andrestoration of brake pressure. It is found desirable, therefore, tocompensate for hydraulic and mechanical l vs in the brake applyingmechanism proper. Preferably such compensation is achieved by utilizinga characteristic of the speedrecovery ydetector mechanism, namely itsability to produce a detection signal somewhat in advance or" the actualattainment of full recovery speed ofthe wheel.v Ther amount of thisanticipation afforded by the detection. mechanism normally is madesubstantially equal to the lag in the brake applying mechanism, therelationship varying somewhat with landing conditions.

Moreover, it is found to be essential to controlled. braking efficiencythat braking pressure not be restored too soon` following release of thebrakes inv response to a skid detection signal. lf the brakes` arereapplied much before the wheel recovers speed in these circumstances, asucceeding skid will tend to develop more quickly than it restoration ofbraking pressure is deferred until substantially full recoveryv of wheelspeed has taken. place. As a. result, over an ensuing period ofalternate relief and premature res-toration of braking4 pressure,braking efciency progressively degenerates to ay low value, Noting thatythe wheel-speed-recovery detector inherently produces a detection signalsomewhat in advance of the wheels attainingy their full recovery speed,the detecting mechanism itself aiords no direct indication of when thelatter occurs. Moreover, the problem is complicated further by the factthat the rate of wheel-speed recovery varies materially with thecoefficient. of friction et the run- Way.

In accordance with further features of the invention it was discoveredthat the foregoing requirement for eicient braking can be satisiied byresolving the complications mentioned into a comfp-aratively simplebasis or criterion of control with respect to timing restoration ofbraking pressure following detection and interruption of an in.- cipientskid condition. For practicalpurposes in the control operation it wasfoundthat runways may be divided into two classes according to whethertheir coefficients of friction, as affecting rate of wheel-speedrecovery, are above or below a certain empirically determined Value..The apparatus itself makes this determination in a given designinstallationon the basis of time lapse between a skid detectionsignaland the succeeding wheel-speed-recovery signal. On. a runwayclassified on the preceding basis as slippery this time lapse will belonger than a certain amount, whereas it will be shorter than thatamount if the coemcient of friction of the runway is sufficiently highto be considered non-slippery. In a typical case the driving line wasvrepresented by a time lapse period or about 0.45 second, although theamount might vary permissibly between 0.4 and 6.6 second, for example,without materially affecting braking eiiiciency, that is, withoutcausing restoration of bralring pressure following a skid, much too soonor too late.

On the basis of the foregoing determination automatically made by thecontrol apparatus the brakes are reapplied substantially immediatelyfollowing reception of the vlheel-speed-recovery signal if the timelapse mentioned is less than the empirically predetermined amount, butif it is more than that amount the control circuit interposes apredetermined short delay, such as 0.75 second in a typical case,between reception of the wheel-speed-recovery signal and restoration oibraking pressure. This further delay afiords additional time for thewheels to recover full speed in the case of slippery runways, on which.the wheels are slower to recover speed. A slowrelease relay having anormal release time of about 0x25 second, but which may vary between ceand 0.6 second due to effects of temperature and line voltage variationson its operation, in the typical case previously mentioned, provides aret'- erence for automatic determination of time lapse between detectionof skid and detection of wheelspeed recovery. lf runways are to bedivided into a greater number of classes than the two mentioned above inorder to attain still greater accuracy in timing the automaticrestoration of braking pressure following a detected skid, for maximumbraking eiciency, then one or more additional slow-release relays or thelike will be required.

A5 a. further feature of the control circuit the release time of therelay just described. is also utilized as a basis of establishing thedesired amount of prolongation of the electric interlock after initialground contact. To the delay period of this relay is added a furtherincrement of circuit delay which has a maximum limit at which theinterlock is removed in any event, but which can be foreshcrtened in theevent o a further actuation oi the recovery detector as in the Case of anormal landing.

The control circuit embodies still another slowrelease relay throughenergization of which such circuit operatively responds to skid signals.lts purpose briey described as follows. Should the skid-and-recoverydetecting mechanism, in shifting from skid detecting position 'toneutral position, overslicot the neutral and rea-ch the recoverydetecting position, a false recovery signal would be produced eventhough the mechanisrn returns to neutral immediately thereafter. Theslow-release feature of this relay prevents the circuit from permanentlyresponding as if to a true recovery signal and returns it to theskidsensitive operating condition.

Still other objects and related features of the apparatus include aso-called fail-safe provision whereby unrestricted control over brakingis restored to the pilot automatically upon occurrenee of any oi variouscontingencies, such as in the event of a failure of electric powerenergizing the control circuit, or in the event of grounding orshort-circuiting of various critical parts of the circuit, or ofsticking of contacts, especially-inv the skid-detector switch. Anothersuch contingency might arise after a predetermined maximum safe intervalfollowing a detected skid, in the event of continued failure for anyreason of the wheel-speed-recovery detector switch to close and restorebraking pressure. One reason this recovery detector switch may fail toreclose following skidding of the wheels may be found in the case of anextremely slippery runway, restoration of wheel rotational speedfollowing skidding beingr extraordinarily slow; another reason ispossible mechanical failure in the recovery detector means itself.

Yet another object is to avoid fluctuations of the solenoid valve due tochattering of the skid detector switch contacts during the incipientskid interval but prior to wheel speed recovery, for example. To thisend the control circuit inm cludes holding relay means which eectscontinuing relief of braking pressure in response to a short orintermittent skid detector signal, such as that caused by bouncing ofthat particular landing wheel or by intermittently recurring slpperinessof the runway upon which it rolls.

Each landing wheel preferably is subjected to separately controlledbraking of the kind indicated and each separate braking control meansembodies the features described.

Further features, objects and advantages of the invention will appearfrom the following detailed description thereof by reference to theaccompanying circuit diagram. The circuit is intended, in effect, tosubstitute for the control circuit disclosed in the copendingapplication of Gordon W. Yarber mentioned above, and to cof operate withthe basic wheel skid and speedrecovery detection means describedtherein, in a manner which will become apparent. lt is believed thatenough has been and will be said generally concerning the detectingmechanism itseli to make clear the operating characteristics thereofnecessary to an understanding of the present improved control apparatus,without need herein for a detailed description and drawings of suchmechanism.

In the preferred and illustrated case the system control operations arefocused upon the solenoid valve I located in the pressure-fluid supplyline I2 extending to hydraulic actuating mechanism of a wheels brakes.Normally the brake uid pressure Valve is positioned to open this supplyline and enable the pilot to vary the actual braking force at will bymetering the pressure of iiuid supplied to the brake-actuatingmechanism. Each wheel is similarly controlled, and each wheel brakesupply line i2 has a similar solenoid valve il) connected therein. Thediagram represents control apparatus for a single wheels brake.

The solenoid valve lll may be of any conventional three-way type suitedfor the purpose; and the valve details per se constitute no part of thepresent invention. When its solenoid is energized, flow ofpressure-fluid through line I2 is cut oli and diverted into a by-pass orreturn line it leading back to the pressure-fluid reservoir, resultingin relief of brakingr pressure. Such solenoid energization isaccomplished by the control circuit to be described herein and waslikewise accomplished by the control circuit disclosed in the copendingpatent application cited above. It is important to note, in either case,however, that the control circuit which energizes the solenoid valveunder certain conditions to prevent skidding of the wheels does notinterfere with `point of initiating skidding.

direct pilot control over braking unless and until he applies excessivebraking pressure to the Thus the valve I0 in general a selectivelyoperated brake disabling means. Moreover, because of the fail-safeprovision, previously mentioned, in no event can the control circuitsfailure to operate, or the occurrence of a defect therein, leave theaircraft wheel without elfective brakes: nor can pressure- Fluid pocketin the brake actuating mechanism and lock the brakes, because valve lllalways returns to the line-open or pilot control position in the eventof apparatus failure.

An indicator light L2, `connected across the valve solenoid, provides avisible indication to the pilot that excessive braking pressure is beingapplied and` is being relieved by operation of the solenoid valve, andyet not for the purpose of warning him that the pilot-metered brakingpressure should be reduced. Rather it indicates that the control circuitis operating effectively, because it is the intention herein that thepilot will usually, or at least can if he so desires, meter or establisha higher braking pressure than is actually needed for maximum brakingbelow the point of skidding, and rely upon the automatic controlapparatus to establish the desired lower eifective average pressure,incipient to wheel skidding, for effecting maximum retardation of theairplane, as previously explained.

The control circuit in the aircraft application should be designed foroperation from the standard 28 volt D. C. or other power supply normallyavailable in airplanes. It should operate satisfactorily despite widevariations in supply voltas between 20 and 30 volts, for example, as mayoccur during a given landing. In the diagram circuit, current normallyat 28 volts is supplied through conductor IB when the main power switchi8 is closed. This switch may be closed manually or automatically bylowering of the landing gear preparatory to landing theairplane-preierably the latter, so that it will require no attentionfrom the pilot who is already occupied with numerous duties. Thisenergizes the conductor 20 to which different parts of the controlcircuit are connected to receive energizing current. Branch conductors26A, 253B and 2d() serve a similar purposey in corresponding controlcircuits (not shown) for the other landing wheels of the airplane, threesuch branch conductors being shown, and used in the case of an airplanehaving four separatelybraked landing wheels.

By way of general introduction, when the onoff power switch I3 isinitially closed or 011, control relays Y and Z become energized, andthis is their normal operating condition, so referred to herein, duringlanding of the airplane. correspondingly, the relays W and X arenormally deenergized, but are subject to energization in tandem byclosure of skid-detector switch contacts F5 corresponding to thecontacts liti and 5 described in the copending patent application citedabove, or by energization of relay V followed by closure of Wheel speedrecovery contacts R (lili, til) at initial ground'contact. The relay Vis normally deenergized, but during the initial phase of landing,commencing prior to the touchdown, this relay is energized by momentaryactuation of a so-called arming switch 22. The relay V and arming switch22, operating in conjunction with the remainder of the circuit, providethe initial electric interlock feature described above, the effect ofwhich is to insure that solenoid valve lil cannot be deenergized until.such time as there is rst an initial ground contact of the landing wheelsufficient to spin the iiywheel and condition the control circuit forcontrol operation, as later explained. Relay V is deenergioed. byrelease of relay Y at a predetermined time following initial closure ofconu tacts R.

In the ground roll, or run, phase of operation of the control circuit,that is, after the initial ground contact effectsl removal of theelectric interlock by deenergization of relay V followed by opening ofrelays W and X if or when re covery contacts R are closed after Yreleases, the solenoidv valve is energized to relieve braking pres sureonly hy reclosure of relay X, which, through its contacts Xe, suppliesenergizing current tov the valves solenoid through conductors Et and Et.On the other hand, during the initial landing phase when the electricinterlock relay V is energized for freeing the wheels preliminary totouch-down, energizing current is delivered to the valves solenoid viaconductor 215, through conductor closed contacts 'Vd of relay V,conductor 3s in series. Therefore, until the electric interlock isremoved by deenergisation of relay V, the solenoid valve is energized toinsure that the brakes will not be applied no matter what may be theoperating condition of relay X. lt is immaterial, therefore, as towhether the pilot, before touch-down, inadvertently sets the brakepressure high enough to cause wheel loot; and shielding when the wheelsrst strike the ground. Such pressure is then rendered ineffectual.Moreover, as mentioned above, in the case of an imperfect landing one ormore of the respective relays W (also X) in the diiierent wheel brakecontrol circuits may perpetuate energisa tion of valve it beyond thepoint of release of relay if', ii recovery contacts R are not closedwhen slow-release relay Y releases in each such circuit.

The control circuit and its details of operation he understood best, itis believed, by descrihn ing it in relation to thev two distinct landingphases, namely the second or ground run landing phase which occurs afterrelease of the electric interlock, and the initial landing phase duringwhich such interlock is effective. Moreover, the second landing phaseentails use of components and functioning thereof which are also basicto the combination and functioning of the circuit components operativeduring the initial landing phase, and .so will be described first,although it will be understood that the actual sequence of these twodistinct phases of operation is the converse of their order ofdescription herein.

Ground run Zending phase It will be understood that the detectormechanism as disclosed in said copending application, includes aninertia member or flywheel by reierence to the speed of which switchcontacts lili, t, designated S herein, are closed automatically inresponse to initiation of wheel skid, and also the switch contacts 656,designated R herein, closed automatically in response to wheel speedrecovery. More specically, the recovery contacts R close whenever theairplane wheel is rotated at a speed which tends to exceed theinstantaneous speed of the reference flywheel, and they close somewhatahead of full recovery of wheel speed to provide a recovery signal whichanticipates full recovery. There is a neutral po sition of the `detectormechanism associated with the flywheel, in which neither of the contactsS or R is closed. This condition obtains when both the flywheel and thelanding wheel are stationary, and also when they are rotatingsubstantially in eynchronisni as, for instance, immediately following arotational impetus irnparted to the flywheel by acceleration of theairplane landing wheel tending to increase its speed ahove that of theflywheel. However, if the landing wheel suddenly decelerates as byinitiation of shielding, the resulting speed differential, in

relation to the flywheel as a reference, causes skid contacts S toclose. The amount or degree of deceleration of the landing wheelrelative to the iiywheel necessary in order to actuate the skid contactsin this manner may be established at a desired or preferred value,depending upon the installation, as explained in said copendingapplication. Moreover, the amount of acceleration of the landing wheelrelative to the flywheel, that is, the degree o recovery of wheel speed,necessary to close the recovery contacts Pt, is similarly subject tochoice in accordance with 0perating requirements. In a typical case,under normal landing conditions, contacts R were set to close by a fewtenths of a second. ahead oi full recovery of wheel speed, such intervalcorresponding approximately to the lag between the recovery si cal andactual application of brakes.

The y W is connected directly conductor and the skid cont cts eh in thediagram is ground contacts S are closed, rei y sed. in ironiediateeffect n of relay X hy connectri rgioed conspeed e tleoending, in a thatlanding wheel. th:^ quicsl.' reopen, relay W re; l hrough a holdingcir-- cuit includ l and series contacts Xo and Za, as later mentioned i.detail.

li wheel recovery release ci hralr v it will he when e wheels lowingsteadily on a noinslipp or comparatively tractionable runway, such as arunway having a friction coeicient above 6.3, for recovery contacts l1?.will he closed correspondingly soon closure el skid following in thatstuation the uicield coil oi relay t n is then direc sttwclooed contactsc J'rounded through the ductor closed contacts l of v coan ductor andthe r; y contacts U"ound n .e a made thro upplyiine to ea )ely the .itscult be fen, relay W also is deeneig shortly It close, as laterexplained. Short-circuit current 9 by-passing the coil of relay Xthrough contacts Yb and Vb is limited by the small resistor 36, such astypically 60 ohms, for instance.

After Wheel speed is recovered, contacts R again open and the circuit isrestored to neutral condition, completing the anti-skid control cycle.The skid contacts S and contacts R remain in neutral or open positionuntil wheel skidding again occurs, whereupon the skid contacts S againclose and relays W and X become energized in the same sequence aspreviously to energize the solenoid valve Ill and again relieve brakingpressure to release the wheels. Thereupon the cycle of operation justdescribed repeats itself. If the pilot meters a steady brake fluidpressure in excess of that which. produces skidding on the particularrunway, the contacts S and R will alternate between open and closedposition, and the circuit will hunt between the skid and wheel speedrecovery conditions, producing an average effective braking pressurebelow skidding, but as high as possible within that limitation, with theresult that the airplane is brought to rest in the shortest possiblelanding run. This accomplishment of maintaining a high average ericctivebraking pressure below skidding is accomplished automatically withoutattention from the pilot, and is a result solely of his application ormetering of what may be termed full effective braking pressure duringthe entire landing run.

It may be that under certain conditions release of skid contacts S maytake place so energetically that momentaryV closure of recovery contactsR results, the flywheel-actuated mechanism overshooting its neutralposition. At that time recovery contacts R, are not required to closebecause wheel speed recovery had not yet occurred. t is important thatsuch momentary false recovery signal be prevented from effecting apermanent or definite change in the operating condition of the circuit,short-circuiting relay X. This problem is conveniently solved byincorporating slow-release means in relay W, such as a conventionalcopper slug which carries induction currents opposing collapse of therelay field, and thereby introducing a short time lapse, such as 0.15second, before the relay releases after its full energizing currentthrough contacts S is interrupted. Hence, although relay X may bemomentarily short-circuited by i'alse actuation of contacts R, relay Wremains unreleased long enough to reenergize relay X, hence itself,through a holding circuit to be described, until such time as a truerecovery signal is received and utilized to deenergize solenoid valvelli for reapplication of braking pressure to the brake mechanism.

Concerning relays W and X which participate jointly in eiiecting theforegoing described braking control operation, it will be noted, aspreviously mentioned, that a holding circuit for relay W is completed byenergization of relay X, including resistor il?, closed contacts Xa, andthe normally-closed lower contacts Za of relay Z to ground at 32. Thus,even ii the closure of skid contacts S is but momentary or is erratic,or intermittent, the holding circuit nevertheless maintains a steadyflow of energizing current to rent in relay W at a minimum valuesuilicient to maintain armature-holding energization thereof, so thatthe contacts of this relay, despite its slow-release feature, Willspring quickly to` the released or deenergized position at a time, forinstance, following opening of the holding circuit when relay X isshort-circuited by actuation of recovery contacts R. A quick release ofrelay W is desirable when it is being energized through its holdingcircuit, as distinguished from its ini-- tial energizing circuitincluding skid contacts S, because if the recovery contacts R chatter orclose intermittently when they should be closed steadily any appreciabledelay in the release of W at that time might cause X to operateerratically.

A conductor 44 directly interconnects `the ground side of the eld coilof relay X With its normally `closed contacts Xb, which in turn areconnected through a bridging conductor 48 directly to the normallyclosed contacts Za` of relay Z, thence to ground at 112. Theseconnections provide a by-pass around current-limiting resistor 38 whichis included in the normal path for energizing current flowing throughthe coil oi' relay X. Faster operation or energization of relay X isthereby obtained, since an initial iinpulse of current passes throughthe relay coil which is greater than the normal energizing currenttherein, but this by-pass is immediately removed and resistor 3Bconnected in the relay energizing circuit upon operation of the relay toopen its contacts Xb.

Certain delayed reactions are initiated in the circuit upon energizationof relay W. Closure of skid contacts .S and resulting energization ofrelays W and X interrupts the normal ilow of energizing current throughcontacts Xd to the coil of relay Y from energized conductor 20. Currentthrough relay Y, now limited by resistor 3d, drops abruptly to a muchlower value, below the amount necessaryifor holding in the relay. RelayY is of the slow-release type, and resistor 34 is selected to be of sucha size that, in conjunction with the slow-release means, such as acopper current-induction slug, in the relay proper, a predetermineddelay, such as normally 0.45 second, is obtained in the actual releaseof relay Y following energization of relay W.

Relay Y is a timing device which determines, from the interval betweenclosure of contacts S and subsequent closure or' contacts R, how quicklybraking should be restored, i. e. valve lli actuated, once contacts R doclose, as will be explained. Its delay interval of 0.45 second is merelyillustrative, but was determined to be optimum in a typical andpreferred installation. In that installation it was an approximatemeasure or the maximum lapse oi' time between a skid signal andsubsequent recovery signal which would normally be expected under runwayconditions `approaching what might be classified as slippery. In otherWords this ernperically determined delay period represented the dividingline in a typical case, between runway conditions requiring immediatereapplication of brakes and those requiring further delay inreapplication of brakes, after the recovery signal. Such delay intervalin that case could vary between 0.4 to 0.6 second as engine speed, hencegenerator voltage varied under different landing conditions representingLa wide range of ambient temperatures such as might be encountered inthe extremes of ying weather. Nevertheless, such variation` canbetolerated` and ll the release delay period of relay Y remains sufcientlydennite to serve an important purpose.

The signicance of the slow-release feature of relay Y, involving the0.45 second delay interval, variable between 0.1i to 0.6 second, forinstance, will now be explained in terms of circuit operation. Afterdevelopment of a skid is initiated on a non-slippery runway, such as onehaving a friction coenicient of more than 0.3, for example, relay Wthereupon being energized through contacts S to eiect interruption ofnormal energizing current to relay Y and initiate its delayed release,recovery Acontacts R will usually be expected to close before the lapseoi the 0.4 to 0.6 second relay Y release delay period. lt has been foundin the illustrative case that braking should be restored, for mosteffective results, almost immediately after the re'cove'rycontacts Rclose, if they close within the period indicated. This is accomplishedby short-circuiting relay X to ground through contacts Yo ofstill-unreleased relay Y, contacts Vb of relay V, and the recoverycontacts R, as previously described. The recovery signal occurringsomewhat ahead of full wheel speed recovery, `as it does with theparticular skid and recovery detecting mechanism, the brakes .are thenactually reapplied, despite mechanical lag, at approximately the instantsuch full recovery inaterializes in (most cases, as desired. l'lullwheel speed recovery is thereby permitted to occur, yet 'controlledbraking is restored immediately thereupon, a type of control which isfound `to give maximum braking efficiency.

However, if the runway is slippery having a friction coefficient oi lessthan 0.3, .for example, then a longer period of time should `be allowedbetween the recovery signal and .actual ireapplication of brakes. Afterabout vone-fourtlfi or .more of the airplanes weight is carried by thelanding wheels niost runways even when wet will act as if of thenon-slippery class on the basis herein described. An icy runway is, orcourse, another matter. rihe circuit determines the requirement or anadditional delay in reapplication of brakes with sufficient accuracy byutilizing the delayed release of relay Y to niark the convenientlyarbitrary, although empirically determined dividing line betweenrequirements for eicient controlled braking on a non-slippery runwayand'those on a slippery runway. It has been found that if the recoverysignal occurs, therefore, beyond the 0.4 to 0.6 second delay interval,then the brakes should not be applied immediately as in the previoussituation, but a delay `should be interposed to permit the wheels moretime in which to regain full or synchronous speed, before the brakes areapplied.

vTo this end the circuit includes further delay means conditioned foroperation by release of relay Y and operated by closure of recoverycontacts R to eifect release of relay X, hence reapplication of brakes,at' a predetermined time later, preferablyv after a'delay of about 0.75second, more or less, in a typical case. Such a means includes the relayZ, release of 'which opens-contacts Za. and deenergises relay X thereby.Relay Z is also of the slow-release type,'but its coil is connectedacross an R-C' delay circuit, and is supplied with a small orYsuiza-holding current from conductor 0 through a large resistor andconductor 52, which gives it a much longer natural .release vperiod thanslow-release relay Y. In the circuit, relay Z may have, and preierablyhas for reasons Alater explained, a 'natural maximum release period oras long as 3 or more seconds. This periodcan be foreshortened, however,by closure of recovery contacts R before expiration of such naturalperiod.

Before release of relay Y the ield coil of relay Z has been energizeddirectly from conductor 20 through the closed contacts Wb of relay W,and conductor 52, by passing resistor 35. Following closure of relay Wby closure of skid contacts S, relay Z is then connected to theenergiaed conductor 20 through the large resistor 35 which limitscurrent through relay Z to a value below its holding current, but suchcurrent, together with the delay imposed by the R-C circuit and with therelease delay inherent in the relay itself, provides the long vreleaseidelay period mentioned. The resistance-capacitance delay or .storagecircuit, including a resistor r, such as 500 ohms, and a condenser c,such as 500 microfarads, connected across the coil kof relay Z, slowlydelivers the normally retained charge of energy on condenser c to relayZ following opening of contacts Wo and materially delays deenergisationand release of relay Z. However, closure of recovery contacts r,occurring before the expiration of relay Zsnatural release delay period,materially accelerates the discharge rate of condenser C by creating aby-pass circuit to ground through the now-- closed contacts Yc and Vb.This greatly diminishes the residual current then flowing 'in the rclaycoil, so that it releases sooner. However, it will not releaseimmediately upon closure of `contacts R,\but even when thus acceleratedrequires, in a typical case, about 0.75 second residual delay for actualrelease to take place. This interval varies somewhat depending upon thetime lapse between energization of relay W and closure of recoverycontacts R after relay Y releases. 'Ihe interval tends to be longer ifini'- tiated immediately after relay Y releases, and shorter if at alater time, but the variation is not great or objectionable. Resistor rImay be varied to adjust this delay interval within limits.

As will now appear, therefore, closure of contacts R, if after relay Yreleases, restores braking pressure in line l2 to the wheel brakes, butdoes so only after an additional delay of v0.75 second, more or less,over and above the inherent hydraulic mechanical lag in .the brakesystem, as desired. Full recovery of wheel speed on a slippery runway isthereby assured and efficient, controlled braking obtained.

In the case of ya slippery runway the period of oscillation or huntingof the circuit between the skid and brake-released conditions will belengthened because of the greater wheel speed recovery time manifestedon a slippery runway as compared with the shorter period required on amore tractionable runway.

If in the extraordinary case a recovery signal from closure of recoverycontacts R is not received before the expiration of the describednatural release period or relay Z to accelerate release of such relay,as mentioned, then the reason may be assigned to some diiiculty orfailure in the'operation of the control mechanism or circuit which wouldprevent contacts R from closing. The same results would obtain by powerfailure in the system, by sticking of skid contacts S following closurethereof, by a short-circuit or ground in the circuit preventing releaseof `relay Y, for instance, or by other difliculties any yof which couldoccur during warfare, for instance. In sho'rt, Vthere are variouspossible sources of aesa'oo.

trouble which might conceivably work to prevent' normal restoration ofbraking pressure through deenergization of solenoid Valve Il), and it isfor this reason that slow-release relay Z with its release delay periodoi' the amount indicated, for example, is employed. r.The naturalrelease delay period oi relay Z in the circuit is preferably selected toexpire at a time beyond the occurrence or the most belated recoverysignal which may be received in any of the expected or predictableoperating or landing conditions of the apparatus. following a skid, therecovery contacts R are not closed within the delay period of relay Z,then something will have been amiss; relay Z will then release and causeopening of the energizing circuit to relay X, thereby deenergzing thevalve solenoid to restore braking. In no event, therefore, is theairplane crippled by absence of braking, and yet the braking is alwaysautomatically controlled as mentioned, un-

ditions, in order to bring the airplane to a speedy halt. Branchconductors 54a, heb and 54e extend from the lower side of the troublelight L1 to the upper contacts Zh of relays Z in the similar controlcircuits associated with the other landing wheels, so that if any of thecontrol circuits K deve-lop trouble the pilot will be appraised thereof.Alternatively, a separate trouble light may be employed in each of thebranch connections 54a, till) and tallo, as well as in the circuitdiagrammed.

Initial landing phase (circuit interlock) After the landing airplaneonce settles steadily on the ground and all the wheels have been or arebeing rotated suiiiciently to spin their respective reference ilywheelsand initiate operal tion of their individual automatic braking controlcircuits to prevent skidding, each such control circuit operates aspreviously described. However, in the process of bringing the airplaneinto its first contact with the runway, referred to as the touch-down,it is possible that the pilot might accidentally apply or meter to thebrakes before and at such touch-down, such high braking pressure thatthe touch-down is accompanied by a skid instead of by spinning of thewheels. Not only is this in itself dangerous because of the possibilityof tire failure and loss of airplane control, but it is furtherdangerous and undesirable in the usual case because the skidding thusinitiated will not be broken by the operation of the automatic controlapparatus. For the latter to operate in order to stop askid the airplanelanding wheels must 'First be rotated to spin the lywheels by referenceto which skid detection signals are obtained. Accordingly, a furtherimportant feature of the present invention resides in the precautionaryprovision` of electric circuit interlock means, including the relay Vand the arming switch 22. This interlock portion of the circuit operatesautomatically and its effect is to insure energization of solenoid valvelil to divert pressure-fluid into by-pass line i4, so that the pilotcannot accidentally apply the brakes until the airplane has actuallyfirst landed. It is further desirable, however, that brake pressure bemade available and the -con--j trol circuit placed into normal operationwith the interlock removed as soon as feasible upon landing or initialtouch-down. Moreover, pro-A To these ends the relay V and the armingswitch Z22 cooperate with other parts of the circuit previcuslydescribed to provide a more dependable and versatile control systemsuccess fully meeting a wider Variety of landing conditions. Ariningswitch 22 is preferably of the momentarily operated type, and maycomprise a simple push-button switch which the pilot presses momentarilypreliminary to landing, or preferably an automatically operated typeswitch which is closed inomentraily during the process of lowering thelanding gear, but following closure of the main power switch i8 in anyevent. Closure of arming switch 22 completes the energizing circuit forthe coil of relay V through the normally closed lower contacts Ya ofrelay Y. The arming switch need be pressed only long enough for thecontacts of relay V to switch positions, and

when that occurs a holding circuit for this relay is created throughconductor 2li, the contacts Vc of relay V, conductor 56, the relay coilitself, conductor 58 and normally closed contacts Ya of ref lay Y, toground. The upper contacts Vd of relai7 V simultaneously close andconnect the circuit conductor 2t to the energized conductor 2i! throughconductors 28 and 3d, thereby energizing the solenoid valve It to removebraking pressure independently of the operating position of relays W, X,Y or Z.

At the same time, the lower contacts Vc: of relay V close to form acircuit including the field coil oi relay Vl' connected to energizedconductor 2d, conductor Gil and the initially open recovery contacts R,to ground. Contacts it remain open until the airplane wheel underdiscussion actually touches the ground and is spun thereby, but whenthis occurs closure of recovery contacts R necessarily results becauseof the nature of the wheelspeed-recovery detector described in saidcopending application. Relay W is then energized. This results inenergization of relay X which forms the holding circuit for relay Wthrough resistor 42 and, because relay Ys current is then greatlyreduced by resistor 34, initiates the delayed release of relay Y tobreak the energizing circuit for relay V at 0.4 to 0.6 second afterinitial ground contact. This delay imposed by relay Y allows time forany minor wheel bounce to be dissipated.

Such a sequence of events, therefore, commencing with closure ofrecovery contacts; R, upon initial ground contact, represents theinitiation of that phase of circuit operation which will culminate indeenergization of relays V, W and and in removal of the interlock andrestoration of normal braking as regulated by the controll apparatus,when the airplane settles steadily upon the runway.

When relay V is deenergized, which occurs once and for all during agiven landing by re lease of relay Y, the only source of energization ofthe solenoid valve It is through continued or subsequent energization ofrelay X. If relay X is not released after relay V is released, then theinterlock effectively continues. It continues` or ,S

absence I7 trol" element' actuated by" said "deceleration-detectingmeans` and'. operablel to"4 alterk the" selection'fromsaidiiist'brake-restoring means to saidl secondbrake-restoring "means upon.expiration oi the? delayperiod of said' controlv element following'suchactuation. thereof'.l

8'. The" automaticl Wheel' bra-lie control. system defined' in claim7Wh'erein the` delay' period or the" rsti brake-restoring' means issubstantially zeroV and the delay period of thesecond brakerestoringmeans is materially greater in order to. allow. appreciableadditionalacceleration of th'eWheel followingactuation of the recovery-detectingmeans.

9J The automatic Wheel brake controlsystem deiined in claim 8; whereinthe timing means comprises a selective timing. device initiatedautomatically in responseto. detection of' deceleration off 'the Wheel!andi, having a natural time periodVv required to, elapse.` afterintiation before selective operation thereof, a iirst meansoperableivvhen actuated to. restore braking` substantially immediately,a second and alternatively operable means, including a delay device,operable When` actuated to restore braking after a prede-- termineddelay., and., means selectively controlled byv. saidi` timing device.andzoperable automatically:` in responseto. the. advance ydetection oi.`recovery: of I'Wh'eelspeed to. actuate: said iirst means or,alternatively, said second means in the event oi' such advance detectionoccurring Within or beyond the natural timing period of said selectivetiming device, respectively.

10. The automatic Wheel brake control system defined in claim 9, whereinthe selective timing means comprises delay circuit means havingalternative switching positions, one such position corresponding toselection of the first brake restoring means and the other such positioncorresponding to selection of the second brake restoring means, andfurther wherein the second brake restoring means comprises delay circuitmeans responsive with predetermined delay after initial actuationthereof to eiiect brake restoring operation of said second means aftersuch delay.

11. The automatic Wheel brake control system defined in claim 9, whereinthe delay device of the second brake restoring means includes delaymeans having a maximum natural delay period after expiration of whichsaid second means immediately operates to restore braking independentlyof an advance detection of Wheel recovery speed.

l2. In an automatic airplane-Wheel brake control system comprising meansfor detecting Wheel deceleration from running speed caused by initiationof skidding of the Wheel, and for releasing the brake automaticallythereupon to interrupt the skid and enable the Wheel to acceleratetoward running speed, such skid-detecting means being of the typerequiring spinning of the wheel initially on landing to condition suchskid-detecting means for operation thereof, and means responsive towheel speed acceleration for detecting approach of the Wheel to runningspeed, but in advance of its attaining of full running speed, initialskid preventing means actuated at will preparatory to landing forpreventing application of the brakes despite all other controlinfluences, means operable to deactuate said initial skid preventingmeans automatically in respense to actuation of said accelerationresponsive means caused by spinning of the airplane wheel during a shortinitial period of landing,

lil)

and timing'means thereafter operableautomaticallyto" restore brakingsubstantially immediately inI response to.` and upon said advancedetection ci fully attained wheel running speed if the lapse of timebetween said detection of deceleration and said advance detection offull speed is shorter' than a predetermined amount, follow-- ing releaseof brakes in response to detection of skidding; and` to restore braking'in responseto` such advance detection, but with predetermined delaythereafter, if said time lapse is greater'than said predeterminedamount.

13. The automatic control system deiined inl claim l2; whereinthe'deactuating means responsive to the acceleration responsive means co.L

prisesv delaymeans having'a predetermined delay' period substantiallyequal in length to therst'- named predetermined delayperiod,initiated-in` responseto actuation ofthe acceleration responmeans. Y

14. In an automatic airplane-wheel brake-conL trol. system includingskid-detecting means requiringspinning of the Wheel initially on landingfor enabling skid-detecting operation of "suchj means, brakek releasingmeansautomatically actuated by said" skid-detecting means When" thewheel begins to skid, and means responsive to Wheel acceleration forrestoring braking after the wheel recovers speed following such removalof brakes; the combination of initial skid preventing means actuated atwill preparatory to landing for preventing application of the brakesdespite all other control influence, and means operable to deactuatesaid initial skid preventing means automatically in response toactuation of said acceleration responsive means caused by spinning ofthe airplane Wheel during a short initial period of landing.

l5. The combination defined in claim lli, wherein the deactuating meansresponsive to the acceleration responsive means includes delay meanshaving a predetermined delay period initiated in response to actuationof the acceleration responsive means upon initial ground contact andoperable to delay deactuation of the initial skid preventing means byfurther actuation vof saidV acceleration responsive means, at leastuntil termination of said predetermined delay period '16. Thecombination dei-ined in claim 14, wherein the deactuating meansresponsive to the acceleration responsive means includes delay meanshaving a predetermined delay period initiated in response to actuationof the acceleration responsive means upon initial ground contact andoperable to delay deactuation of the initial skid preventing means byfurther actuation of said acceleration responsive means, at least untiltermination of said predetermined delay period, said deactuating meansincluding further delay means having a materially longer predeterminedmaximum delay period initiated in respense to actuation of saidacceleration responsive means and operable to effect operation of saiddeactuating means automatically upon termination of such maximum delayperiod if not operated earlier in response to said further actuation ofthe acceleration responsive means after initial ground contact andspinning of the landing wheel.

17. In an automatic Wheel brake control system comprising means fordetecting Wheel deceleration from running speed caused by initiation ofskidding of the wheel, and for releasing the brake automaticallythereupon to interrupt the skid and enable the Wheel to acceleratetoward running speed, and means for detecting thereafter the Wheelsapproach to running speed in advance of its full recovery thereof;apparatus controlling restoration of braking upon detection of wheelspeed recovery, said apparatus comprising timing means automaticallycomparing the time interval between actuation of saiddeceleration-detecting means and subsequent actuation of saidrecovery-detecting means with a predetermined reference period initiatedby actuation of said deceleration-detecting means, and means controlledby said automatic timing means and by said recovery-detecting means forrestoring braking substantially immediately upon actuasensing means forselectively deactuating said w brake means controlled by rotation of thelanding Wheel to remove and restore braking thereof automatically inresponse to skidding of the Wheel and recovery of Wheel rotationalspeed, respectively, arming means operable preparatory to landing of theairplane to prevent braking of such landing Wheel on initial groundcontact thereof and thereby prevent initial skidding of such Wheel, anddisarming means controlled by the Wheel, disabling said arming means topermit actuation of said brake means automatically in response torolling ground contact of the wheel upon landing.

19. The system combination defined in claim 18, wherein the disarmingmeans is controlled by the skid-sensing means to permit actuation of thebrake means initially only in response to the `landing Wheel reachingsubstantiallyv full running speed upon initial ground contact thereof.

GORDON W. YARBER. HARRY H. HOWELL. RUSH F. CHASE.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 2,232,750 Wilson Feb. 25, 1941 2,393,031 Eksergian Jan. 15,1946 2,468,199 Hines Apr. 26, 1949 2,515,729 Morrison July 18, 1950

